Stationary Barotropic Flow Induced by a Mountain over a Tropical Belt

Fredrick H. M. Semazzi Department of Meteorology, University of Nairobi, Nairobi, Kenya

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Abstract

Stationary solutions in the presence of bottom topography are computed for a tropical atmospheric belt. A primitive divergent barotropic model is assumed. An iterative time-averaging method proposed by Edelmann (1972b) is used for the purpose of suppressing gravitational oscillations and synoptic transient perturbations. The spatial finite differencing scheme is analogous to the one adopted in the Mintz-Arakawa two-level general circulation model and documented by Gates et al. (1971). Time integration is performed with the leapfrog scheme using a time stop of 10 min. The initial state consists of realistic zonal flow and the initial geopotential height of the free surface is determined from the wind by assuming steady-state

conditions.

First. we obtain quasi-stationary solutions using an idealized mountain profile over a 36-day integration. The results are used to interpret stationary solutions in the case involving real African orography. When an idealized mountain profile is used, a quasi-stationary equatorial trough is obtained over the mountain. Stationary solutions associated with the orography of the African continent show a height field distribution qualitatively in agreement with the observed quasi-stationary equatorial trough over the African continent land mass during the month of April. Thermal influence dominates in the formation of the equatorial trough in the lower levels of the troposphere. Further aloft, dynamic control becomes significant enough to explain the stationary of the equatorial trough, throughout the year.

Abstract

Stationary solutions in the presence of bottom topography are computed for a tropical atmospheric belt. A primitive divergent barotropic model is assumed. An iterative time-averaging method proposed by Edelmann (1972b) is used for the purpose of suppressing gravitational oscillations and synoptic transient perturbations. The spatial finite differencing scheme is analogous to the one adopted in the Mintz-Arakawa two-level general circulation model and documented by Gates et al. (1971). Time integration is performed with the leapfrog scheme using a time stop of 10 min. The initial state consists of realistic zonal flow and the initial geopotential height of the free surface is determined from the wind by assuming steady-state

conditions.

First. we obtain quasi-stationary solutions using an idealized mountain profile over a 36-day integration. The results are used to interpret stationary solutions in the case involving real African orography. When an idealized mountain profile is used, a quasi-stationary equatorial trough is obtained over the mountain. Stationary solutions associated with the orography of the African continent show a height field distribution qualitatively in agreement with the observed quasi-stationary equatorial trough over the African continent land mass during the month of April. Thermal influence dominates in the formation of the equatorial trough in the lower levels of the troposphere. Further aloft, dynamic control becomes significant enough to explain the stationary of the equatorial trough, throughout the year.

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